Intervertebral disks often become damaged creating discomfort for a patient. It is known that partial or total removal of a damaged disk may alleviate some of this discomfort but may limit the natural function of the spine. Thus, nowadays following the removal of a damaged, natural intervertebral disk or a damaged nucleus pulposus of a natural intervertebral disk, implants or prosthesises are placed in the intervertebral space between the two vertebra adjacent to the site of removal. The aim of implanting such implants is to bring about as natural state as possible, particularly duplicating the original height of the intervertebral disk and consequently restoring the original distance between two adjacent bodies of the vertebra. Furthermore, movements of adjacent bodies of the vertebra relative to one another should be able to be carried out with minimal hindrance of their natural function. For this purpose the retention of the ability to move when bending forward or backward, i.e. the flexion and the extension of the bodies of the vertebra as well as the lateral bending of the intervertebral bodies within the natural limits, is desirable. It is also desirable that the natural ligaments and muscles along the spine are left substantially intact, to further stabilize the movements of a mechanical replacement for a intervertebral disk.
Intervertebral implants for maintaining the displacement between vertebrae adjacent to a removed intervertebral disc and for allowing the movement of vertebra with respect to one another are known in the art. For example, U.S. Pat. No. 5,556,431 to Büttner shows an implant comprising a bottom cover plate and a top cover plate, the external surfaces of which can be placed on, respectively, adjacent vertebra as well as a joint provided between the cover plates. This joint substantially consists of a spherical convex joint part and, two matching joint shells connected with the cover plates, so that the cover plates can polyaxially pivot relative to one another.
One disadvantage of this known construction is, that although the two cover plates are each connected with a joint part, the joint parts are not held together. This requires the implant to be assembled by the surgeon creating a possibility for erroneous assembly. Further, it creates a possibility that the implant will fall apart prior to and during implantation, risking damage to surrounding tissue and risking loss of parts.